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The Centaur, in Greek mythology, is a creature related to both basal instincts and wisdom (lust, or in the context, perverseness). They are connected to the god Dionisius, god of carnal desires (sexual drive) and carousing.

Chiron, by contrast, was intelligent, civilized and kind (...) A great healer, astrologer, and respected oracle, Chiron was said to be the first among centaurs and highly revered as a teacher and tutor.

This could be connected to the fact that Ryosuke is displaying his great knowledge or wisdom, by assimilating the content of the dictionary.

It was a pun, he "read" from the dictionary Decisiveness (Ketsudanryoku in japanese), while centaur is pronunced Kentaurosu.
Probably they decided to do this pun from the origin of the legend of Centaur, a bastard son of a Greek God whom got kicked from the Olymp (like Lisara was from the demon world)